Control for focal spot in X-ray generators



Z. J. ATLEE ETAL Filed March 3, 1942 CONTROL FOR FOCAL SPOTS IN X-RAYGENERATORS Feb. 1. 1944.

0 0 0 .w v w w w mm R o m% w a 9 H 015 J 4 m m a F m P m .0 l Mm J w W Zw W m a r i HN a me m wmmw WW amwam w E H 0 H0 wpwwvm E w a wfi xwm G NMW 5 a Ma m m5 LQSWkKB-WQAK I w m o w w m w w w m w o Patented Feb. 1,1944 CONTROL FOR FOCAL SPOT IN X-RAY GENERATORS Zed J. Atlee, Elmhurst,and Frank R. Abbott,

Berkeley, Ill., assignors to General Electric X- Ray Corporation,Chicago, 111,, a corporation of New York Application March 3, 1942,Serial No. 433,208

7 Claims.

Our present invention relates in general to electronics and has moreparticular reference to the operation of X-ray generators, the inventionapplying specifically to the control of an X-ray generator in order tomaintain the focal spot configuration as nearly as possible in optimumcondition throughout the operating range of the generator.

An important object of the invention is to overcome the space chargeefiect in an X-ray generator when in operation at low voltages, so as toallow substantial current to flow between the anode and cathode of theorder of the current flowing when the generator is operated at highvoltages; a further object being to overcome the space charge effect,when the generator is in operation at low voltage, by applying apositive biasing potential on the cathodecup with respect to'theelectron emitting filament of the cathode.

A further important object of the invention is to provide for theoperation of an X-ray generator so as to maintain, throughout theoperating range of the instrument of the generator, a focal spot patternas'nearly as possible, and preferably uniformly, in accordance with theoptimum or ideal focal spot pattern, the ideal pattern'comprising alarge, sharply defined area,

on the drawing an X-ray generator ll com'prising a cathode l3 and aco-operating anode l5 enclosed in suitable hermetically sealed envelopeand the ideal generatorloperating conditions be.-.. 3

ing such that the spot maintains a constant shape and definition throughthe operating range of the generator.

Another important object is to control the focal spot pattern in an,X-ray generator by applying positive anode bias; ,a further objectbeing to obtain control of focal spot distribution by positively biasingthe cathode focusing cup with respect to the electron emitting filamentof the cathode; a still further object being to regulate or adjust thepositive biasing potential as an inverse function of generator operatingvoltage so that maximum positive bias is ,applied when the generator isin operation at low voltages applied between anode and cathode, thebiasing potential being reduced proportionally when the operatingvoltage is increased.

These and numerous other important objects, advantages and inherentfunctions of theinvention will become apparent as the invention is morefully understood from the following description, which, taken inconnection with the accompanying drawing, discloses a preferredembodiment of the invention.

' e r n 1 1 h? draw n e-i'is a diagrammatic representation of means It.The cathode includes a focusing cup I! and electron emitting filamentsl9 mounted in the cup in position to discharge electrons toward and uponthe anode l5. The anode 15 comprises a tubular member 2| having a cavity23 facing toward the'cathode and has a target 25 at the bottom of thecavity in position to re.- ceive electrons emitted by the cathode whenthe tube is in operation.

It should be understood that X-ray generators are conditionedforoperation by exhausting from the envelope means It substantially allgaseous and other impurities by evacuating the envelope, as by means ofamolecular exhaust pump, and finally sealing the envelope in evacuatedcondition. X-rays are generated at the target 25 as a result ofimpingement thereon of electrons emitted as a stream 26 by the cathodefilaments is when electrically excited. Such electrons are impelledtoward the target under the influence of the driving force providedthrough the application of operating voltage applied between thefilament and anode from an external source, the focusing cup I! ofthe'cathode, through electrostatic action, serv ing to confine anddirect the electron stream in a desired path toward the anode target.The shape and configuration of the cathode cup, to some extent,determines the surface area or focal spot'of the target 25 within whichthe electron stream impinges upon the target. Impingement of theelectron stream upon the target 25 results in the generation of Xerayswhich pass from the target laterally through the walls 2liof the anodestructure, which are;

preferably formed with a window 21 to facilitate the passage of X-raystherethrough. The X- rays thus transmitted from the target form ,a beamor cone 29, the sectional shape of which is determined by the pattern ofthe anode focal spot.

It is desirable that the sectional configuration of the X-ray beam 29 beuniform throughout the operating voltage range of the generator, and itis further desirable that the boundaries of the beam 29 be sharplydefined and that X-ray intensity be uniform throughout the beam. Thesefactors, in turn, are determined by the uniformity and definition of thetarget focal spot. We have noted that the size, configuration. andmarginal definition of the focal spot of any given' X-ray generatorchanges as the operating voltage is varied between anode and cathode.For example, as shown in the upper row of diagrams in- Figure 5, thecharacter of the focal spot in a given X-ray tube may improve as thetube operating voltage increases, the focal spot having desirablecharacteristics when the tube is operating at relatively high voltagesand having impaired characteristies at lower voltages. Thisphenomenais-due apparently to changes in electron focus on the targetresulting from changes in the operating voltage, it being understoodthat electrons have optical characteristics analogous to light rays andthat the focused pattern of the electrons on the target depends upon theshape of the electron source, which, in the illustrated embodiment,comprises a pair of parallel filaments, the shape of the focusing cupl1, and the voltage applied between filaments and target. In thisconnection, the top diagram in the left-hand column of Figure 5represents a sharply focused electronic pattern or image of theelectronemission elements on the target, while the other diagrams of Figure 5represent the filament patterns in varying derees of soft focus.

In order to improve the character of the focal spot pattern when thetube is in operation at lower voltages, we provide for the applicationof a positive bias potential between the electron emitting filaments I9and the focusing cup l1, and we have discovered that electron flow maybe controlledin this fashion to produce substantially the same focalspot pattern when the generator is operating at low voltage as isproduced without biasing the cathode when the generator is operating ina high voltage range.

The three lower rows of diagrams in Figure 5,

respectively, illustrate the focal spot patterns I developed through theoperating range of the generator when biasing the cathode cup, respectively, at 300, 600 and 900volts, and it will be noted that thecharacter of the focal spot pattern is progressively improved byincreasing the biasing potential while the tube is in operation at lowvoltage; but that the character of the focal spot pattern deterioratesas the biasing potential is increased when the generator is in operationin the higher voltage ranges. The particular generator which producedthe focal spot patterns illustrated in Figure 5 was a generator builtfor normal operation at voltages between filament and anode of the orderof 100 kv. p., and it will be noted that when operating at normalvoltage, the application of a positive biasing potential has little, ifany effect upon the character of the focal spot pattern.

When in normal operation at a voltage of the order of 100 kv. p. andwithout cathode bias, the anode current which may flow is limited bythespace charge between anode and cathode, and the focal spot has theusable pattern shown in the second diagram in the first row of Figure5.' .As the operating voltage'is increased, the.'eifectof the spacecharge is minimized, with the result that the focal spot pattern isimproved, as shown in the third and fourth diagrams in the first row ofFigure 5. If, however, the tube is placed in operation at a voltagesubstantially below its rated voltage, the effect of the space chargeincreases to such an extent that the focal spot pattern deteriorates,becoming sharp and narrow.

The graph shown in Figure 3 illustrates the space charge limitations ofthe generator operating at low voltage of the order of 60 kv. p. withthe filament carrying 4.5 amperes. The curve shows the variation ofanode current as cathode bias is increased. From this curve, it will benoted that saturation is attained at 39 milliamperes tube current, withabout 800 volts positive cathode bias; and. there is relatively littleincrease in tube current above biasing potential of 350-400 volts, atwhich point in the curve the anode current is of the order of 33milliamperes. Three hundred fifty to four hundred volts, with anodecurrent at 33 amperes, may therefore be selected as the upper limit forstandardizing cathode biasing procedure, although, of course, the rangemay be increased,- if desired. The curves shown in Figures 3 and 4 weremade from data obtained in operating the particular tube which producedthe patterns shown in Figure 5. Corresponding curves, however, may beobtained for any generator and, of course, may vary from the curvesshown, depending upon the optical and electrical characteristics of theselected generator.

The curves in Figure 4 show the relationship between cathode biasingcurrent in milliamperes to cathode filament current in amperes forvarious bias Voltages, with the tube in operation at 60 kv. p. The-uppercurve represents conditions with cathode bias at 1000 volts. The middlecurve represents conditions with cathode bias at 660 volts. The lowercurve represents conditions with cathode bias at 330 volts. These curvesare of interest, since the upper curve approaches 200 milliamperesrequired in the cathode biasing circuit at 4.5 amperes in the cathodefilament, that is to say, the operating condition for the curvein Figure3. This means. simply that with 600 volts bias at a filament, current of4.5 amperes, 200 milliamperes pass to the cathode, releasing 120 wattsof energy, inaddition to the normal cathode heat generated due tofilament heating alone, which, at 4.5 amperes of filament current,amounts to about 40 watts. The biasing trans former, therefore, shouldbe several times larger than the filament heating transformer. The focalspot patterns shown in Figure 5, with no cathode bias, are satisfactorywhen the generator is in operation at voltages in excess of kv. p. Withcathode biased at 300 volts, the spot pattern is only slightly improvedwhen the generatoris in operation at 60 kv. p., changed when thegenerator is operating .at.100 kv. p., and shows deterioration'whenthegenerator is operating at voltages in excess of 100 kv. p. With'thecathode biased at 600 and 900 volts,- the spot pattern improves withthegenerator operating at voltages below 100 when the generator is inoperation at the higher voltages between cathode and anode.

In order to control the X-ray generator for operation with satisfactoryfocal spot pattern throughout its ,full operating range, a positive biasof at least 600 volts is needed on the cathode when the generator is inoperation within avolt age range up to voltages of the order of 100kv.p.

is substantially unkv. p. but deteriorates A bias of this magnitude,however, when the tube .'is inoperation at higher voltages. across anodeand vcathode, results in the production of focal spot patterns ofdiminished size so that'for "most effective operation,'the bias should,when the generator is in operation at the high voltage range, be eitherremoved entirely or diminished in order to reduce tube current tothereby avoid overloading the anode heat dissipating capacity of thetube, the following schedule being a'satisfactory guide illustrating thedesirable variation in bias (expressed in terms of tube current) atvarious voltages within the operating range of the generator:

Bias rating in terms of anode current Operating voltage (kv. p.)

ment l9 and the anode 2|, the primary winding 39 of the transformer 3|being connected, by conductors 4| and 4 3, with a suitable source ofgenerator operating power.

The control system also includes filament exciting means which maycomprise a filament exciting and biasing transformer 65, a portion d1 ofthe secondary winding 49 of which is connected to energize the cathodefilament l9 through suitable conductors 31 and 5|. Biasing potential maybe applied between the filament l9 and the cathode cup l1 by means ofthe transformer 45, a portion 53 of the secondary winding 49 of whichmay be connected to the filament, through the conductor 5!, and to thecathode cup |1 through a conductor 55, which conductor preferablyincludes switch means 51 for con-trolling the biasing circuit. Thisswitch 51 preferably comprises a shiftable blade electrically connectedwith the cathode cup H, the blade being movable in order to make contactwith the winding 53 in order to apply cathode bias, or to make contactdirectly with the conductor 5| in order to remove the cathode bias byelectrically connecting the oathode cup with the filament circuit. Theswitch 51 may be controlled by a solenoid 59 operable, when energized,to throw the switch blade 51 into bias applying position. When thesolenoid 59 is de-energized, suitable spring means may be utilized tothrow the switch 51 into position connecting the cathode cup directlywith the filament circuit. Preferably, the switch operating meansincludes a stem insulated, as at 69, to assure electrical isolation ofthe switch from the operating circuit of the solenoid 59. The primarywinding 6| of the transformer 45 is electrically connected, as by meansof the conductor 23 and a conductor 63, with a suitable externalelectrical power source.

The generator H, the transformers 3| and 15, the switch 51 and itsoperating solenoid 59, and the several circuits for connecting theaforesaid control elements with the generator, are preferably allenclosed in and insulated from a container or grounded casing 65; andthe mid point of the Winding 33 of the transformer 3| may beelectrically connected with the grounded casing 65, as by means of aconductor 61.

. The transformers 3| 'and'45 and the switch control solenoid 59 may beenergized in any suitable or preferred fashion, but we prefeerablyutilize a supply system comprising an autotransformer and controlswitches having rheo'stats, which are preferably enclosed in andinsulated from'a. grounded container 69, which may be located remotelyfrom the generator casing 65. The. autotransformer may comprise asuitable winding 1|, one end of which may be connected by means of theconductor 13 with one side of a suitable external power source, theconductor means i3'leading to the primary windings oithe transformers 3|and being also electrically connected to the power source through theconductor 13. The other side of the power source may be connected,.through a. conductor 15 and a control switch 11, to the winding 1|intermediate the ends thereof. The winding 1| may be connected, by meansof an adjustable connection 19 through a control switch 8| and anadjustable rheostat 83, to the conductor 4| forenergizing the primarywinding of the transformer 3|. The winding 1| likewise may be connected,through an adjustable rheostat 85, to the conductor 53 for the purposeof energizing the filament heating and cathode biasing transformer Theswitch operating solenoid 59 may be connected to the winding 1| througha switch 81 and may also be connected to a remote portion of the winding1| through means for adjusting the voltage applied to the solenoid, andsaid means may conveniently comprise the adjustable connection 19. Byclosing the switch 81, the solenoid 59 will be energized to throw thebiasing switch 51 into position applying a bias upon the cathode. Thisbias, because of the adjustable rheostat 83, may be varied as a functionof filament current. The bias will be maintained so long as the solenoid59 remains energized. The solenoid, however, may comprise a deviceadapted to operate at a predetermined voltage in order to throw theswitch to a position discontinuing the bias on the cathode. Theadjustable contact 19 is a control for determining the operatingvoltage, applied between anode and cathode, of the tube through thetransformer 3|, and by utilizing this control means 19 in the operatingcircuit of the solenoid 59, the system may be adjusted to operate theswitch 51 for the removal of the cathode bias whenever the element 19 ismanipulated to increase the operating voltage of the generator above apredetermined value, thereby automatically accomplishing the control ofthe generator in accordance with the teachings of our present invention.It will be apparent, however, that the control of the bias need notnecessarily be interlocked with the control of generator operatingpotential but may be had by separate means if desired.

It is thought that the invention and its numerous attendant advantageswill be fully understood from the foregoing description, and it isobvious that numerous changes may be made in the form, construction andarrangement of the several parts without departing from the spirit orscope of the invention, or sacrificing any of its attendant advantages,the form herein disclosed being a preferred embodiment for the purposeof illustrating the invention.

The invention is hereby claimed as follows:

1. The method of operating an X-ray generator in which the cathodecomprises a focusing cup and an electron emitting element which consistsin applying positive bias on said cup with respect to said element whenthe generator is'in operation at low voltage and diminishing theconsists in positively biasing said cup with respect to said element asan inverse function of the operating voltage of the generator.

3. The method of operating an X-ray generator in which the cathodecomprises a focusing cup and an electron emitting element which consistsin positively biasing said cup with respect to said element of thegenerator when in operation at relatively low voltage and reducing thebiasing eifect as the operating voltage is increased.

4. The method .of operating an X-ray generator in which the cathodecomprises a focusing cup and an electron emitting element which consistsin maintaining uniform focal spot pattern throughout the operatingvoltage range of the generator by positively biasing said cup withrespect to said element inversely with respect to generator operatingvoltage throughout the operating voltage range of the generator.

5. The combination, with an X-ray generator comprising an anode, acathode including an electron emission element and a mounting head,means to apply generator operating voltage between the emission elementand the anode, and

means for adjusting said operating voltage, of biasing means forapplying a positive bias potential to said head with respect to saidemission element, and means operable to control said biasing meanswhereby to reduce the intensity of bias progressively as the operatingvoltage is increased.

6. The combination, with an X-ray generator comprising an anode, acathode including an electron emission element and a mounting head,means to apply generator operating voltage between the emission elementand the anode, and means for adjusting said operating voltage, ofbiasing means for applying a positive bias potential to said head withrespect to said emission element comprising a biasing circuit, and meansto control the biasing circuit to vary the bias inversely with respectto variation in the generator operating voltage.

7. The combination, with an X-ray generator comprising an anode, acathode including an electron emission element and a mounting head, andmeans to apply generator operating voltage between the emission elementand the anode, of energizing means for delivering element energizingelectrical current to said emission element, biasing means for applyingpositive bias on said head with respect to said emission element, meansfor adjusting the energizing current delivered to said emission element,and means for varying the bias as a function of the variation infilament current.

ZED J. ATLEE. FRANK R. ABBOTT.

